Sometimes clean air is a luxury. A recent study linked outdoor air pollution to 2.1 million deaths annually around the world. Now a new study finds that indoor pollution from unsafe household cookstoves may be even worse, killing as many as 4 million people every year. The toll is especially high in the developing world, where families are exposed on a daily basis to smoke, particulate matter, carbon monoxide and toxic chemicals from burning wood, dung, coal and agricultural waste, often in homes with little to no air circulation.

The new study comes via Airfilters.com in support of the Global Alliance for Clean Cookstoves, a public-private partnership that hopes to equip 100 million households with safer cooking and heating devices and fuels by the year 2020. The study finds that 42 percent of the world's population is affected by indoor smoke and pollution on a daily basis. In many of these homes, the average concentration of the deadly airborne particulates known as PM10 reaches up to 5,000 micrograms per cubic meter. That's 100 times the levels of PM10 exposure recommended by the World Health Organization. It gets even worse the closer people are to the fires, where PM10 concentrations can reach an astonishing 50,000 micrograms.

The health implications of this exposure are wide, varied and debilitating. Chronic obstructive pulmonary disease (COPD) has been heavily linked to unsafe cookstoves in the developing world. More than 2 million children under the age of 5 die each year from acute lower respiratory infections, at least half of which are caused by cookstove exposure. Other health risks include low birth weight for children and lung cancer, tuberculosis, cardiovascular disease and cataracts in adults. In most cases, medical treatment for these conditions is not available.

The toll of indoor air pollution

All told, the study finds that indoor air pollution is responsible for up to 4 percent of the world's entire disease burden. This causes 4 million deaths a year — or, as the alliance calculates, one every eight seconds. The worst-hit areas include rural Africa, India, China and other Asian countries, and island nations around the world.

Luckily, the alliance says there are solutions in hand. Advanced biomass cookstoves and forced-air-stoves can reduce air pollution, while others use less fuel and therefore produce fewer emissions. Solar cookstoves have been used successfully in some regions, while adding chimneys to homes can dramatically reduce indoor pollution. Part of the challenge is convincing families that these new solutions, which don't necessarily employ traditional methods, can meet their cooking and heating needs. Another challenge is cost: cookstoves can cost as little as $15 but up to $150, which many families can't afford. The alliance, which is funded by public donations, helps to get stoves into many of these households.

For more information on cookstoves and indoor air pollution, check out this infographic: